Conventionally, a valve timing control system is widely known, which controls a phase adjustment unit and a lock unit by collaboration of a control valve and a command unit. The phase adjustment unit adjusts a rotation phase between a crankshaft and a camshaft. The lock unit can lock the rotation phase.
For example, in a system disclosed by Patent Literature 1, a phase adjustment unit and a lock unit are driven by hydraulic fluid such as oil. Specifically, the phase adjustment unit has an advance side oil pressure chamber and a retard side oil pressure chamber, between rotors rotating with a crankshaft and a camshaft, respectively. A torque generating direction is opposite between the advance side oil pressure chamber and the retard side oil pressure chamber, when changing the rotation phase in response to introduction of oil. The phase adjustment unit adjusts the rotation phase based on the flow in/out of oil to the advance side oil pressure chamber and the retard side oil pressure chamber. The lock unit has a lock pin between the rotors of the phase adjustment unit. Oil pressure is applied to the lock pin from an unlock oil pressure chamber to which oil flows in/out through the advance side oil pressure chamber. The lock unit unlocks the rotation phase when an oil pressure more than or equal to an unlock pressure is impressed to the lock pin, in a state where the rotation phase is locked in which the oil pressure impressed to the lock pin is less than the unlock pressure. In the system disclosed in Patent Literature 1, the flow in/out of oil to the advance side oil pressure chamber and the retard side oil pressure chamber is controlled by operating a control valve based on a current value set by the control command unit, to drive the phase adjustment unit and the lock unit.
In the system of Patent Literature 1, the current value for starting the impression of operation pressure more than or equal to the unlock pressure to the unlock oil pressure chamber, in the state where the rotation phase is locked, may be set to a value that can change the rotation phase by introducing oil to the advance side oil pressure chamber. However, if the oil pressure of the advance side oil pressure chamber increases rapidly before the unlocking, the rotation phase may be rapidly changed to affect the lock pin between the rotors. Then, a subsequent valve timing adjustment may be affected.
In the system of Patent Literature 1, the current value is set for starting the applying of the operation pressure low enough not to change the rotation phase as an unlock pressure. Thereby, it is possible to avoid a faulty operation of the lock pin that is caused by a change in rotation phase before the unlocking.